In signal processing, aliasing refers to a signal distortion effect that causes different signals to be indistinguishable (“aliases”) when sampled. If the sampling frequency of a signal is not sufficiently high, a signal will not be perfectly reconstructed, and artifacts (errors) may occur when the signal is interpolated from the samples taken.
According to the Nyquist-Shannon Theorem, a signal can be perfectly reconstructed from a sampled version of the signal if the sampling rate is greater than two times the bandwidth of a bandlimited signal being sampled—or, in other words, when the Nyquist frequency (half the sampling rate) is greater than the highest frequency of the signal being sampled. However, in practice, perfect reconstruction of signals from these samples is unattainable because filters do not operate ideally, and some amount of aliasing is often unavoidable.
Further, it is not always possible to increase the sampling rate (for example, due to certain standards in an industry) to account for higher frequencies in an information signal. If one or more frequencies in an information signal are above the Nyquist frequency, these frequencies may disturb the information content during sampling. Frequencies above the Nyquist frequency are not interpreted properly and cause interference, which causes the output signal to appear to an end user (e.g., on a screen) as an aliased signal.
Additionally, even if all frequencies in an information signal are below the Nyquist frequency, the envelope around a sampled signal may not be perfectly representative of the actual signal. For example, if a sampling frequency is slightly more than the Nyquist frequency, the envelope around the sampled signal can appear (e.g., to an end user) to be modulated, even though no modulation is applied to the signal. This effect may be referred to as “envelope modulation” or the “envelope effect.”
This envelope effect can be especially pronounced when test patterns, which are designed to introduce stress into a display system, are used. Test pattern frequencies often approach half the sampling rate and may even exceed half the sampling rate in some instances, in order to evaluate the display under test.
Accordingly, what is needed are systems and methods to compensate for this “envelope effect” and improve overall image quality.
Features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.